1. Field of the Invention
The present invention relates to a liquid crystal display apparatus, and more particularly to a construction for adjusting the contrast of the same.
2. Description of the Prior Art
In X-Y dot matrix liquid crystal display apparatuses, the liquid crystal has the cumulative response characteristic that it responds to the sum of the effective voltages of an applied voltage pulse for several ten to hundred milliseconds. Interference generally known as crosstalk, tends to occur in which ON or OFF display information for an attention pixel is affected by display information for other pixels on the scanning side electrode and the signal side electrode that intersect each other at the attention pixel. To prevent such interference, the so-called voltage equalization method is employed to drive the liquid crystal. In such liquid crystal display apparatuses, adjusting the contrast, i.e., difference in pixel display density between ON and OFF, is essential to compensate for the temperature characteristic and viewing angle characteristic of liquid crystal, variations in threshold voltages between liquid crystals, and the viewer's liking with respect to the contrast.
FIG. 1 is an electrical circuit diagram of a portion of a typical prior art construction. The voltage from a DC power supply 1 is divided through four resistors R and one registor Rx so that voltages V10-V15 are derived through impedance converters A1-A5 to obtain a voltage Vop between the voltages V10 to V15, the voltage Vop being varied by means of a variable resistor Ry thereby adjusting the contrast of the liquid crystal. The voltages V10-V15 are supplied to an X-Y dot matrix liquid crystal display apparatus through a driving circuit composed of semiconductor integrated circuits, etc., to drive the liquid crystal by the voltage equalization method.
In the voltage equalization method, the relationship between the voltage Vop and the effective voltage value VON applied to pixels selected for energization is expressed by equation (1), while the voltage VOFF applied to non-selected liquid crystal segments that are set to OFF state is expressed by equation (2). ##EQU1## where B is the bias ratio, and D is the number of scanning side electrodes.
FIG.2 is a graph showing the relationship of the voltages VON and VOFF relative to the light transmittance of liquid crystal. The voltage VOFF is set at a value close to the threshold voltage Vth at which the energization of the liquid crystal just occurs.
To increase the contrast, it is desirable to determine the bias ratio B in such a manner as to maximize the ratio of the voltage VON to VOFF. The bias ratio at which VON/VOFF is the maximum is given as: ##EQU2##
The temperature characteristic of liquid crystal is such that, as shown in FIG. 3, a higher voltage VON is necessary at lower temperature to energize the liquid crystal to the desired contrast. Also, the voltage VON at which the desired contrast is achieved varies from one liquid crystal display apparatus to another, and further, the desired contrast varies according to the taste of the viewer. Therefore, a construction that can adjust the voltage VON is needed.
Usually, the upper limit value of the voltage Vop is determined by the withstanding voltage of the driving circuit composed of semiconductor integrated circuits to which the voltages V10-V15 are supplied. On the other hand, the voltage VON is set at the highest value considered necessary at low temperature, and further, the number of scanning side electrodes, D, is predetermined. Using these values, the bias ratio B is calculated from the aforementioned equation (1). In equation (1), if the voltage Vop is small, or if the value D is large, the bias ratio B cannot be set to the optimum value shown in equation (3).
In the prior art, the bias ratio B is set at a relatively small value in consideration of the upper limit of the voltage Vop so that a large value which becomes necessary can be provided as the voltage VON. When the bias ratio B is set at such a small value, the ratio VON/VOFF approaches 1, presenting the problem of decreased contrast.
In the prior art construction shown in FIG. 1, the bias ratio B is fixed to a value expressed by equation (4) below, while the variable resistor Ry is varied to vary the voltage Vop, thereby adjusting the voltage VON. In equation (4), the resistance values of the resistors R and Rx and the variable resistor Ry are represented by the same reference sign. ##EQU3##
To sum up, in the prior art, the bias ratio B is fixed to a relatively small value in consideration of the upper limit value of the voltage Vop under the condition that requires the highest voltage VON; therefore, the problem is that the bias ratio B is significantly shifted from the bias ratio specifically shown by equation (3), thus resulting in poor contrast.